The Modified Chemical Vapor Deposition (MCVD) is one of optical fiber manufacturing methods. In the MCVD, a clad is firstly formed, and then a core is formed inside the clad.
To describe the conventional MCVD in more detail with reference to FIG. 1, a deposition tube 1 made of silicon oxide is put on lathe (not shown), and then soot generation gas pertaining to halide such as SiCl4, GeCl4 and POCl3 is blown into the deposition tube 1 together with oxygen gas while rotating the deposition tube 1. At the same time, a flame providing unit 2 such as a flame burner or a torch is periodically reciprocated along the axial direction of the deposition tube 1 to heat the deposition tube 1 higher than 1600° C. so that the soot generation gas flowed into the tube 1 is sufficiently reacted with the oxygen gas.
Whenever the flame providing unit 2 reciprocates once, the oxidization reaction of the soot generation gas as expressed in the following Reaction Formula 1 is generated in a part of the deposition tube 1 right above the flame, thereby generating soot 3 composed of fine particles. As the flame providing unit 2 advances, the soot 3 is deposited on an inner surface of the deposition tube 1 at an area which is not yet heated, by means of the thermophoresis.
Reaction Formula 1SiCl4+O2→SiO2+2Cl2GeCl4+O2→GeO2+2Cl2
The layer of soot 3a deposited on the inner surface of the deposition tube 1 is sintered by the heat of the flame providing unit 2 adjacently followed and becomes a transparent glass layer having a predetermined thickness.
The soot forming, depositing and sintering processes are continuously repeated according to a predetermined process condition, and thus a plurality of clad layers and core layers are formed in a direction of the central axis of the deposition tube 1, thereby making an optical fiber perform. At this time, because the clad and the core should have different indexes of refraction, composition of the soot generation gas flowed into the deposition tube 1 is controlled according to the refractive index profile of the optical fiber perform to be made. As shown in FIG. 2, the optical fiber perform made as above has a deposition tube 7 on the outermost position, and a clad 6 having a diameter (D) and a core 5 having a diameter (d) inside the deposition tube 7.
An optical fiber is manufactured through the collapsing process and the drawing process of the optical fiber preform. Transmission through the optical fiber manufactured as above is conducted in the wavelength range of 1310 nm to 1550 nm. In order to ensure reliable signal transmission through the optical fiber, an optical loss of the optical fiber should be controlled lower than a predetermined level in the above wavelength range.
The optical loss, which is most essential for the optical fiber, is composed of the Rayleigh scattering loss caused by of density difference and constitution difference of the optical fiber preform, the ultraviolet absorption loss according to electronic transition energy absorption in atom level, the infrared absorption loss according to energy absorption during lattice vibration, the hydroxyl group absorption loss due to vibration of hydroxyl group and the macroscopic bending loss.
However, in case of the optical fiber drawn from the optical fiber preform manufactured by the MCVD, among the above-mentioned various losses, the hydroxyl group absorption loss and the Rayleigh scattering loss are meaningfully caused, so these losses should be suitably controlled.
More specifically, when making an optical fiber preform using the conventional MCVD, a small amount of moisture exists in the deposition tube 1 as impurities. This moisture may be flowed into the deposition tube 1 together with the soot generation gas. In some cases, moisture derived from the combustion reaction in the flame providing unit 2 may be dispersed through a surface of the deposition tube 1 or minute leaks.
The moisture existing in the deposition tube 1 is physically or chemically absorbed on the surface of the soot 3 as shown in FIG. 3 during the execution of MCVD, thereby generating Si—OH bond. However, since the deposition and sintering of the soot 3 are achieved approximately at the same time in the conventional MCVD, the pores of the soot layer 3a, which is possibly used as a dispersion route for removing impurities, are clogged due to the sintering of the soot layer 3a, so the removal of the hydroxyl groups included in the soot 3 is nearly impossible.
As well known in the art, the hydroxyl group chemically bonded to Si in the soot 3 causes absorption loss due to the vibration of hydroxyl group in the wavelength of 1385 nm among the wavelength range for optical signal transmission, thereby deteriorating the optical signal transmission characteristic. Thus, the optical fiber is conventionally not used for optical fiber transmission in the wavelength range of 1340 nm˜1460 nm around 1385 nm.
In order to use all the wavelength range of 1280 nm to 1620 nm, an optical loss in the wavelength of 1385 nm due to the hydroxyl group (OH) in the optical fiber should be lower than 0.34 dB/Km, which is an average optical loss at a wavelength of 1310 nm.
In case of the silica optical fiber, the core composed of germanium dioxide and silicon dioxide has a Rayleigh loss of about 0.28 dB/Km caused by the density and constitution difference of its material itself. Thus, if the absorption loss due to the hydroxyl group is controlled below 0.06 dB/Km (=0.34 dB/Km−0.28 dB/Km), the optical fiber can be used in the wavelength range of 1280 nm˜1620 nm. By using the theoretical calculation, the concentration of hydroxyl group (OH) in the optical fiber should not be more than 1 ppb in order to control the absorption loss due to the hydroxyl group lower than 0.06 dB/Km. However, when only two hydroxyl groups exist on the surface of a particle of the soot 3 having a diameter of 1 μm, the concentration of hydroxyl group comes up to 30 ppb and 0.75 dB/Km, when being converted into ppb concentration and optical loss value. Therefore, it has been considered that the problem of absorption loss due to the hydroxyl group in the wavelength of 1385 nm is hardly solved.
It is known that an OH-free single mode optical fiber may be fabricated by using OVD (Outside Vapor Deposition) as disclosed in U.S. Pat. Nos. 3,737,292, 3,823,995 and 3,884,550, and using VAD (Vapor Axial Deposition) as disclosed in U.S. Pat. Nos. 4,737,179 and 6,131,415. However, as for MCVD, the technique to manufacture an OH-free single mode optical fiber is not yet reported except U.S. Pat. No. 5,397,372. In addition, U.S. Pat. No. 5,397,372 discloses a technique to manufacture an OH-free single mode optical fiber by using a plasma heat source, which is free from oxygen, but its applicability and industrial merit is very doubtful.
On the other hand, in the process of manufacturing an optical fiber preform using the conventional MCVD, chlorine gas is generated as a by-product by the oxidization reaction of the soot generation gas. In addition, the chlorine gas generated as a by-product is again reacted with moisture, which is an impurity existing in the deposition tube 1, so hydrogen chloride is also derived as another impurity.
However, the chlorine-related impurities such as chlorine gas or hydrogen chloride gas may remain in the preform in the shape of chlorine atom or molecule while the optical fiber preform is formed in the deposition tube. If the chloride remains in the optical fiber preform, the difference of refractive indexes of the core and the clad becomes increased, thereby causing that the Rayleigh scattering loss in the wavelength range of 1310 nm˜1550 nm is as a whole increased (J. Non-Crystalline Solids, vol.195, 1996, pp. 176–179).
In addition, since the sintering of the soot layer 3a is conducted at a high temperature over 1600° C. in the MCVD as mentioned above, the solubility of chlorine contained in the soot layer 3a is decreased, thereby possibly causing fine foams in the optical fiber preform. These fine foams act as a factor of causing a microscopic bending loss of the optical fiber.
Thus, it is required in the MCVD to remove not only hydroxyl groups but also chlorine impurities remaining in the optical fiber preform.